I've got the 32gb 4S and I have roughly 15.5 gb available. I have 1464 songs, 7 videos, 942 pictures, and 112 apps, just to give you the full rundown.
I also have a 160gb iPod Classic that has all my music and videos on it. Like many have said before me, it just depends on what you want on your phone. I didn't want all of my music on there. I've got about 7gb of music. I went through my entire collection and picked out my (apparently) favorite 1464 songs and synced only those. But it was easier for me to do knowing that I have the backup of my full collection on my iPod. When in doubt, I always go for the higher storage, especially with something like an iPhone that doesn't have expandable storage. You'll be stuck with whatever you get, so if you have a question about it, I'd err on the side of caution and get the bigger storage.